I already play some FPS games with a head-tracker (shown in the first post). Usually the head-tracking only affects your view, and doesn't control aiming at all (the mouse still aims as usual). e.g. If I turn my head right, the crosshair is now on the left side of the screen.
While I think this is a cool idea, wouldn't playing an FPS with this really mess up your neck?
Looking around a virtual environment is one thing, aiming is another,
Check out the video linked in the first post - when it's disabled, the 1st person gun model stays in the the centre of the screen (except when sprinting, it disappears), and when enabled, it moves around the screen as the view is rotated. Make sure you've got annotations turned on so you can see the enabled/disabled box up the top left.